The invention relates to an oscillator arrangement including a resonant circuit having two poles via which an oscillation generated in the oscillator arrangement can be extracted and including a differential amplifier formed by two emitter-coupled transistors having two collector electrodes of which each one is connected to one of the poles of the resonant circuit, and having two base electrodes cross-coupled to the poles of the resonant circuit.
In the circuit technology for the processing of high-frequency electromagnetic waves it is common practice to use voltage-controlled oscillators, particularly in phase-locked loops. For the integration of such oscillators on a semiconductor it is preferred to use circuits which operate in a push-pull or differential mode in order to minimize the current loop with as regards the frequency-determining resonant circuit because extremely large reactive currents flow in this current loop. This is independent of the fact whether the current loop is disposed within the semiconductor body or whether parts of the resonant circuit are situated outside the semiconductor body. Moreover, a current loop which is as small as possible, i.e. a conductor arrangement which is as compact as possible, in which said reactive currents enclose a minimal area, reduces the radiation of high-frequency power and, conversely, reduces the susceptibility of the oscillator arrangement to external interference. Such electromagnetic interference primarily arises owing to electromagnetic fields or via the power lines and manifests itself as common-mode interference. Furthermore, such a construction of the oscillator arrangement provides a satisfactory value for the signal to phase-noise ratio, because the signal amplitudes are added to one another arithmetically across the elements of the differential amplifier, while the noise is summed quadratically.
In a simple form the oscillator arrangement of the type defined in the opening paragraph basically comprises an emitter-coupled differential amplifier having cross-coupled base and collector electrodes. The collector electrodes are interconnected via the resonant circuit, while the base electrodes are connected to the collector electrodes of the respective other amplifying element of the differential amplifier. As a result of this cross-coupling a positive feedback is obtained, which reduces the damping of the resonant circuit to such an extent that oscillations are produced. One of the elements of the differential amplifier is then alternately conductive. Owing to the correspondingly strong excitation the amplitude of the voltage across the elements of the differential amplifier can become so large that these elements become saturated. However, upon saturation the oscillation amplitude is limited abruptly via the resonant circuit. This leads to a high harmonic content of the oscillation produced by the oscillator arrangement. Moreover, as a result of mixing processes which then occur, low-frequency noise components may be modulated onto the oscillation thus produced and thereby deteriorate the signal to phase-noise ratio.
From EP 0 746 092 A1 an oscillator arrangement of the type defined in the opening paragraph is known, by means of which it can be attempted to circumvent the aforementioned drawbacks. The oscillator described in said document comprises a differential amplifier stage which includes two transistors having their emitter electrodes both connected to ground via a current source. The transistors have their collector electrodes connected to a supply voltage terminal via a collector resistor each. Each of the collector electrodes of the transistors is connected to the base electrode of an emitter follower transistor, which emitter follower transistors have their collector electrodes coupled to the supply voltage terminal and which have their emitter electrodes connected to output terminals on which the oscillation generated in the oscillator arrangement is available. Each of the emitter follower transistors has its emitter electrode connected to the base electrode of one of the transistors of the differential amplifier stage, in such a manner that via each of the emitter follower transistors the collector electrode of one of the transistors is coupled to the base electrode of the respective other transistor of the differential amplifier stage. In this way positive feedback is obtained. The frequency-determining elements are connected between the collector electrodes of the transistors of the differential amplifier stage. In order to power the emitter follower transistors their emitter electrodes are connected to current sources which operate with respect to ground.
In this oscillator arrangement known from EP 0 746 092 A1 the collector-base potential of the transistors of the differential amplifier stage is larger than in the oscillator arrangement without the emitter follower transistors and the collector potential of the emitter follower transistors cannot become smaller than the supply voltage. However, in said oscillator arrangement the onset of saturation of the emitter follower transistor of the differential amplifier stage, or of the respective transistor of the differential amplifier stage which is turned on, causes the amplitude of the generated oscillation to be limited because the base potential of the transistors of the differential amplifier stage can still rise noticeably above the value of the supply voltage and also the base potential of the emitter follower transistors can rise far above the value of the supply voltage. Moreover, the use of the emitter follower transistors and their direct current supply leads to further low-frequency noise components, which cause a further modulation of the generated wave and thereby deteriorate the signal to phase-noise ratio.